As one of semiconductor device substrates, there is an SOI substrate having a silicon layer (which may be referred to as an SOI layer hereinafter) formed on a silicon oxide film as an insulator film. This SOT substrate has characteristics such as a low parasitic capacitance or a high radiation-proof capability since the SOI layer in a wafer surface layer portion serving as a device fabrication region is electrically separated from the inside of the substrate due to a buried silicon oxide film layer (a BOX layer). Therefore, effects such as a high-speed and low-power consumption operation or soft error prevention can be expected, and this substrate appears promising as a high-performance semiconductor device substrate.
As a typical method for manufacturing this SOI substrate, there is a wafer bonding method or an SIMOX method. The wafer bonding method is a method for, e.g., forming a thermal oxide film on a surface of at least one of two single-crystal silicon substrates (silicon wafers), then closely attaching the two wafers to each other via the formed thermal oxide film, performing a bonding heat treatment to increase bonding strength, and thereafter reducing a film thickness of one wafer by, e.g., mirror polishing to manufacture an SOI substrate.
On the other hand, the SIMOX method is a method for ion-implanting oxygen into a single-crystal silicon substrate, then performing a high-temperature heat treatment (an oxide film forming heat treatment) to react the implanted oxygen and silicon, thereby forming a buried oxide film layer.
Specifically, for example, oxygen ions (O+ in general) are first implanted into a single-crystal silicon substrate heated to approximately 500° C. from one surface. As ion implantation conditions, generally, an accelerating voltage is 150 to 200 keV, and an oxygen dose amount is classified into a high dose that approximately 1×1018 to 2×1018 atoms/cm2 or more is implanted and a low dose that a smaller amount is implanted. After the oxygen ions are implanted, the implanted oxygen (an oxygen ion implanted layer) can be changed into an oxide film (a buried oxide film layer) having a thickness of approximately 220 nm to 440 nm by performing an oxide film forming heat treatment at a high temperature (1300° C. or above in general) in an inert gas atmosphere containing 1% or below of oxygen.
As compared with the wafer bonding method, such an SOI substrate manufacturing method based on the SIMOX method has advantages that a manufacturing process is simple, manufacture is possible at a relatively low cost since manufacture from one single-crystal silicon substrate is possible without requiring two wafers, film thickness uniformity of an SOI layer is excellent since an oxygen implantation depth can be controlled by using implantation energy, and others. Therefore, an SOI substrate manufactured based on the SIMOX method (an SIMOX substrate) is expected as a material of a fully-depleted transistor in which an SOI layer is, e.g., 50 nm or below, and others.
However, this SIMOX substrate has a problem that the buried oxide film layer has a dielectric breakdown voltage inferior to that of the thermal oxide film formed based on the wafer bonding method and also has a high density of threading dislocations that occur in the SOI layer, thereby degrading device characteristics.
Thus, to reduce occurrence of the threading dislocations in such an SOI layer, there is developed a low-dose SIMOX technique that performs oxygen ion implantation with a low dose amount (2.5×1017 to 5.0×1017 atoms/cm2) to manufacture an SIMOX substrate based on discovery of a fact that a threading dislocation density is dependent on an oxygen dose amount (Japanese Unexamined Patent Publication (Kokai) No. H4-264724). Further, when carrying out the SIMOX method with such a low dose amount, to obtain a buried oxide film layer having an excellent dielectric breakdown voltage and an SOI layer having a low threading dislocation density, a dose amount of oxygen ions must be approximately 3.5×1017 to 4×1017 atoms/cm2 (Journal of Materials Research, Vol. 8, No. 3, 1993, pp. 523-534), and a range of such a dose amount is known as a dose window. In this case, a thickness of a buried oxide film layer formed by performing an oxide film forming heat treatment after oxygen ion implantation is limited to approximately 80 nm to 90 nm.
Furthermore, as a technique that provides an SOI substrate having a structure in which a film thickness of a buried oxide film layer is increased and flatness of a buried oxide film interface is improved with a very low rate of occurrence of pinholes (regions that are not locally oxidized in the buried oxide film layer), there is, e.g., ITOX (Internal Thermal Oxidation) processing for performing an oxide film forming heat treatment to form a buried oxide film layer and then carrying out an oxidation heat treatment in a high-temperature oxygen atmosphere to grow the buried oxide film layer (Japanese Unexamined Patent Publication (Kokai) No. H7-263538).
Giving a more specific explanation, according to this ITOX processing, after ion-implanting oxygen ions under a low-dose condition, an oxide film forming heat treatment is performed at 1300° C. or above for several hours in an atmosphere where an oxygen partial pressure is less than 1% to form a buried oxide film layer, and thereafter a heat treatment is further effected at 1300° C. or above for several hours in an atmosphere where an oxygen partial pressure is approximately 70%, thereby growing the buried oxide film to increase a film thickness thereof. Performing this ITOX processing enables obtaining effects that a dielectric breakdown voltage of the buried oxide film layer can be improved, an interface of an SOI layer/the buried oxide film layer can be flattened, and surface roughness of a surface of the SOI layer can be improved. Moreover, this processing also has an advantage that the SOI layer is consumed when the oxide film is grown on the surface of the SOI layer, thereby obtaining the thin-film SOI layer.
Further, as an applicative technique of the low-dose SIMOX method or the ITOX processing, an attempt to amorphize a crystal to facilitate growth of a buried oxide film layer by ion-implanting a low dose amount of oxygen ions at a room temperature or ion-implanting oxygen and an element other than oxygen, e.g., silicon, after ion implantation of a low dose amount of oxygen ions is often made (e.g., Japanese Unexamined Patent Publication (Kokai) No. S63-217657, Specification of U.S. Pat. No. 5,930,643, and others).
According to such ITOX processing or other applicative technique, a dielectric breakdown voltage of the buried oxide film layer can be improved compared to that in the regular low-dose SIMOX method, but the obtained buried oxide film layer is restricted to have a large thickness.
In recent years, in an SOI substrate, a thick buried oxide film layer may be demanded to increase a dielectric breakdown voltage and, on the other hand, a thin buried oxide film layer may be demanded to improve heat conduction at the time of a device operation or to be used as a back gate oxide film, and hence a technique that can manufacture an SOI substrate in which a film thickness of a buried oxide film layer is reduced to a desired buried oxide film layer thickness of, e.g., 80 nm or below is required.
However, a thickness of a buried oxide film layer formed based on the conventional SIMOX method is restricted to a fixed range as explained above, and a continuous and uniform thin buried oxide film layer cannot be obtained.
It is to be noted that a thin buried oxide film layer means a buried oxide film layer having a thickness of 80 nm or below hereinafter unless stated.